In many industrial processes involving multiphase fluid mixtures where the components and mixtures may be stationary, moving in bathes or flowing continuously, there are needs for accurate and inexpensive phase concentration monitoring methods and means. It is also often desirable that these methods and means have the capability of working on-line with the processes.
A number of methods have been used in the past to monitor the phase concentration of multiphase fluid mixtures. Generally these methods seek to find a specific property which is significantly different for the phases. The value of this property for the mixture will then depend on the phase concentration. By measuring this property one would be able to find the phase concentration. Examples of the specific property are electrical properties (i.e. conductivity or capacitance), density, viscosity, absorption of light or absorption of radioactive radiation.
Precise and safe measurement of conductivity or capacitance requires relatively simple instrumentation. Thus, methods based on conductivity or capacitance have been widely used in practice for measuring phase concentrations not only in solids-liquid systems but also in gas-liquid, liquid-liquid and three-phase systems.
Examples of conductivity or capacitance based devices are disclosed in U.S. Pat. No. 4,266,425 to Allport et al., U.S. Pat. No. 3,523,245 to Love et al.
The prior art systems described above, however, have a few of major drawbacks. Electrical conductivity based methods are very sensitive to the variations in the electrical conductivity of the liquid phase of the multiphase fluid mixture. For example, the electrical conductivity of an aqueous slurry may increase by more than 50 times with the addition of 2.5% by weight of salt (NaCl) to the aqueous phase. When the conductivity of the liquid phase changes substantially with time, the conductivities of both the slurry mixture and the liquid phase are required in order to calculate the solids concentration. But the on-line measurement of the conductivity of the liquid phase in a slurry mixture is generally difficult due to the requirement of phase separation. Electrical conductivity based methods are also generally difficult to apply to multiphase fluid mixtures having very low electrical conductivity. The capacitance based methods can be applied only to multiphase fluid mixture where the continuous phase is nonconductive. In the case with aqueous slurries, the high electrical conductivity of the aqueous phase interferes with the dielectric measurement.
In determining the water content of oil/water emulsion mixtures, prior art systems have a significant limitation because of the fact that the electrical properties of water-continuous and oil-continuous emulsions are quite different even if the water content is identical. Prior art systems have also failed to provide methods or means for determining phase composition in fluid mixtures with more than two phases. This is because that different sets of phase compositions may result in similar conductivity or capacitance measurements. Most of prior art systems failed to give accurate measurements when the concentration of the disperse phase in a two phase fluid mixture is low.
The limitation of conductivity or capacitance based methods is attributed to the limited information obtained at a single frequency of excitation alternating current (AC) signal. One known value of mixture conductivity or capacitance is insufficient to determine the phase composition when both the phase composition and the electrical properties of one of the phases in the mixture are unknown.
In certain industrial processes, such as dense medium separation of coal and mineral ores and grinding circuits in mineral processing industry, it is desirable to monitor the average particle size of suspended fine particles in an aqueous slurry under the condition of high solids concentration. At present there are no simple commercially available on-line particle size monitors capable of this measurement. The conventional method of measuring particle size distribution is to remove samples from the streams of interest and to perform screen analyses on these samples. However, screen analysis can provide a reasonably accurate determination of particle size distribution above about 45 microns. There are three commercially available on-line particle size analysers based on ultrasonic attenuation, a scanning laser microscope and a reciprocating caliper. However, these analysers are not suitable for use in slurry mixtures where the average particle size is below 45 micron or the solids concentration is high or the fluid medium is not transparent.
Froth flotation is widely used for concentrating minerals, or other valuable constituents, from their ores or other raw materials. Minerals are separated from gangue particles by taking advantage of their differences in hydrophobicity. These differences can occur naturally, or can be controlled by the addition of a collector reagent. Froth flotation generally involves the use of air injection through a slurry that contains water, minerals and gangue particles within a vessel. Dispersed air bubbles attract the hydrophobic valuable minerals and carry them upward to the top of the flotation cell, whereupon they form a froth bed or froth layer which contains and supports pulverised mineral. The froth is then scraped or permitted to flow over the lip of the cell to effect the separation. The thus concentrated mineral bearing froth is collected and further processed to improve the concentration of desired minerals. The pulp may be further processed to recover other valuable minerals.
On-line measurement of process parameters is a prerequisite for froth flotation process control. Whereas some process parameters can be monitored on-line with cost effective and reliable measuring devices, the effective on-line monitoring and optimal control of froth flotation processes are still far from being achieved because of the strong inertia of the flotation process, a still inadequate knowledge of suitable variables for the on-line monitoring of the process efficiency and the lack of appropriate on-line measurement instrumentation.
The froth phase in a froth flotation process has a number of characteristics, including bubble size, stability, mobility, solids content and water content. The effects of operating conditions such as reagent type, reagent dosage, water chemistry, pulp level, feed flowrate and aeration rate are reflected in the froth characteristics.
The characteristics of froth layer are related to flotation grade and recovery. In view of the difficulty in the direct measurement of the froth characteristics, it is desirable to use other froth properties that can be easily on-line measured as monitoring tools and are closely related to flotation grade and recovery.